The present invention broadly relates to the art of treating metal surfaces for example, ferrous, zinc or aluminum, to improve the properties thereof and more particularly, to an improved composition and method for treating metal surfaces to produce an adherent corrosion resistant coating thereon which is receptive to organic or siccative coatings.
Environmental regulations directed to a curtailment in the level of discharge of environmentally objectionable compounds to waste systems has occasioned a substitution of conventional chromium and phosphate containing treating chemicals in the metal treatment industry with alternative compounds devoid of chromium compounds. For example, U.S. Pat. No. 4,017,334 discloses an aqueous treating composition for aluminum coating containing phosphate, fluoride, titanium and tannin as active coating constituents. U.S. Pat. No. 4,054,466 discloses an aqueous tannin containing composition for metal treatment. U.S. Pat. Nos. 3,682,713 and 3,964,936 disclose aluminum treating compositions containing zirconium and fluoride.
In the treatment of aluminum surfaces, and particularly the surfaces of drawn and ironed aluminum beverage containers, it is important to provide the surfaces of the container with a protective corrosion resistant coating which is substantially colorless in nature and does not impair the taste characteristics of the food or beverages coming in contact with the coating. It is also important that the coating be adherent and receptive to subsequently applied finishes such as paint, varnish, lacquer, etc. to the coated surface. In normal practice, after treatment of the aluminum container, the exterior of the can is decorated and overvarnished on the sidewalls thereof but the exterior bottom of the container receives no organic finish. Accordingly, the only protection afforded to the exterior bottom of the container is the chemical coating.
The qualities required of a coating are many and vary in importance depending on the end use to which the coated article is put. Of concern are:
1. Adhesion of the coating to the metal surface. PA1 2. Adhesion of subsequently applied finish (paint, varnish, lacquer, etc.) to the coated surface. PA1 3. Corrosion resistance of the coated but unfinished surface. PA1 4. Corrosion resistance of the finished surface. PA1 5. Color or colorless nature of the coating. PA1 6. Taste characteristics imparted to food or beverages in contact with the coating or finish. PA1 7. Brightness of the coating. PA1 8. Uniformity of the coating. PA1 9. Coating thickness required to obtain minimum acceptable qualities. PA1 10. Formability of the coated metal article. PA1 11. Etching or other distortion of the metal surface appearance.
In addition to coating quality, the stability of the concentrate and diluted treating bath compositions, the simplicity of process control requirements and energy considerations are of concern to the process operator.
It is also conventional practice after the containers are filled with a beverage such as beer, for example, and sealed, to pasteurize the sealed containers in order to destroy bacteria. This pasteurization process conventionally comprises immersing the filled and sealed cans in water heated at about 150.degree. to about 160.degree. F. for a period of about 30 minutes. The pasteurization treatment does not effect the overvarnished sidewalls of the container but the unvarnished exterior bottom of the container has in many instances undergone severe distortion during pasteurization which is highly objectionable.
It is also conventional for quality control to subject spot samples of the chemically treated containers to a high temperature test to make certain than an adequate chemical coating has been formed thereon. This test usually comprises placing a treated container in a muffle furnace at 1000.degree. F., for a period of 5 minutes. Evidence of a satisfactory coating is visually ascertained by the formation of a dark gold color. Coatings of the type heretofore known have in many instances failed to produce a satisfactory visual color change during the muffle furnace test to enable accurate quality control determination.
It is also desirable in the chemical treatment of such containers that the chemical coating produced is substantially colorless to avoid detracting from the subsequently applied decorative coatings and varnish. Many of the coating systems in accordance with prior art practice result in coatings of a light yellow color which is objectionable, particularly, when the treatment of the containers in the coating solution is prolonged due to line stoppages or the like.
The aqueous acidic coating composition and method of the present invention overcomes many of the problems associated with prior art compositions and practices achieving a substantially colorless, adherent corrosion protective coating on aluminum surfaces which is receptive to subsequently applied organic finishes and which composition and method is effective for forming a coating of the requisite thickness in comparatively short time periods thereby achieving increased throughput and efficiency in metal processing.